A tower of a tall structure such as a wind turbine can easily reach 115 m in height. Such a tower is usually constructed of a number of pre-fabricated tower sections which are transported to the onshore or offshore wind turbine site and assembled there. A tower section comprises an essentially cylindrical ‘shell’ that tapers slightly toward the top. A tower section is generally made of steel, and may have a length of 10-30 m or more, for which a typical diameter might be between 2.0 m and 7.0 m.
Ideally, tower sections would be connected vertically through their shell walls, so that any loading forces would be transferred directly from one shell to the next. However, this is not feasible, and the tower sections are usually connected by pairs of flanges. Generally, a flange is welded to the upper and lower ends of the tower section, depending on its position in the tower. The lower flange of one tower section is connected to the upper flange of a lower tower section, usually by bolts inserted through matching pairs of bores or through-holes in the flanges. To minimize stress in such a flange connection, the flanges are formed essentially at right angles to the shells. The bores are generally evenly spaced about the circumference of each flange in a ‘bolt circle’. For optimal load transfer between the tower sections, the bolt circle diameter (BCD) should be as close as possible to the tower section diameter.
The tower sections must be transported horizontally from the manufacturing site to the wind turbine site. A problem arising during transport is that the weight of a tower section causes the shell and the flange to distort, and their ideal circular cross-sections may become oval, making it impossible to connect that tower section to one another. For this reason, a thick, robust flange may be attached to the tower section in order to ensure that the tower section retains its shape during transport. However, such a thick and rigid flange adds considerably to the costs of manufacture, especially since wind turbine towers are becoming larger, and is also associated with problems during tower assembly. For example, it is difficult to correct any misalignment of the bores owing to the inherent rigidity of the flange. Furthermore, should the thicker flange be in any way distorted during transport, such a distortion cannot be rectified, and the entire tower section must be scrapped.
A thinner or ‘soft’ flange, using less material, can considerably reduce the manufacturing costs of a tower. Such a ‘thin’ or ‘soft’ flange could be supported in some suitable manner during transport to avoid shell and flange distortion. However, a flange connection between tower sections using these thin or ‘soft’ flanges is less reliable compared to a flange connection using thick flanges. To improve the structural strength of a ‘soft’ flange, additional material is used in the transition area between the protruding horizontal flange and the vertical shell. Since the extra material exhibits a curved surface in the smooth transition from horizontal flange to vertical shell, it may be referred to as a ‘radius transition’. While the radius transition can improve the strength of the flange, thus making it possible to decrease the tower section diameter and thereby reduce transportation costs, it is also associated with a number of disadvantages. A washer or nut requires a level contact surface at right angles to the bolt, so that the through-holes or bores for the bolts must be arranged at a considerable distance from such a radius transition. The region of the radius transition is essentially a ‘forbidden’ transition zone in which no bolts may be placed, so that the bolt circle diameter is constrained by the need to form the flange with such a radius transition. However, the stability of the flange connection between tower sections decreases with increasing distance between the bolts and the shell.
In one approach to this problem, the thin flange connection by be strengthened using additional flat ring segments fastened to the lower flange using the bolts, in order to decrease the stresses in the weld by decreasing the amount of deflection of the bottom flange. To be effective, such flat ring segments must be relatively thick and wide. Furthermore, their effectiveness is limited by having to place them outside of the radius transition between flange and shell. As a result, the shell may still suffer damage in the weld area.